Conventional crossflow cooling towers are presently in widespread use and generally comprise a relatively narrow vertical fill section with initially hot water being fed from an overhead source and the air being drawn therethrough from air inlets at the side of the tower. As the water descends in an even distribution along the vertical fill section, the cooling crossflow air currents intersect the descending water in a heat exchanging relation. Subsequently, the cooled water is collected in a water basin below, while the hot, moist air is discharged into the atmosphere.
In a crossflow cooling tower, since there is no necessity for the air to make radical changes of direction in the fill and the air inlet is spaced along the entire height of the fill, the overall air pressure losses are usually less than those of a conventional counterflow tower. Hence, air can be more easily passed through the tower, such as by powered fan.
Unfortunately, a crossflow cooling tower is inherently less efficient with respect to heat transfer than a counterflow tower based on a unit of fill. Another disadvantage of the crossflow cooling tower is that the water is loaded onto the top of the relatively thin crossflow fill. There is a maximum water load beyond which the water will not redistribute effectively because it will start gushing in a steady stream through the tower. When this maximum water load is exceeded in a crossflow tower of the film fill type, the water will not cling to the fill, leading to relatively poor heat transfer between the air and water. Also, resistance to the transversely flowing air is substantially increased requiring excessive fan power or a larger structure. This problem of water loading cannot be effectively overcome by widening the fill in the direction of air flow because there is a limiting factor on cooling efficiency relative to the thickness of the fill. A major factor in this limit is that the resistance to air flow for the longer air path through the fill disproportionately increases in comparison to the advantages to be attained by easing the above water load problems.
These crossflow cooling towers, as well as counter flow cooling towers, generally employ various varieties of splash-type fill sections consisting of elongated bars of a specific configuration for dispersing the descending released water. More recently, film-type fill sections have been developed which have proven substantially more efficient than splash fill sections. These typically corrugated film fills generally consist of a series of thin, opposed sheets formed of synthetic resin materials in which the water passes along the sheets in a "film". Although the film fill sections may be as much as five times as efficient as splash fill sections, their substantial cost differential and higher resistance to air flow has prevented total supplanting of the latter.
A hybrid approach is disclosed in U.S. Pat. No. 3,917,764 which combines the advantages of the counterflow and crossflow cooling towers, as well as combining the fill assemblies with both splash fill sections and film fill sections. Specifically, as shown in FIG. 1, that patent describes a cooling tower 10 with a sloped film fill section 11 having an incline principal plane 12 formed of a number of sheets mounted for the passage of gas and liquid. This sloping film fill section spreads the liquid gravitating onto its upper surface into a thinner, more uniform film on the lower surface. Splash-type fill 13 is disposed inboard and/or outboard of the sloping fill which when combined with corrugated and other types of film fill result in a fairly efficient cooling tower arrangement with relatively low air pressure drop.
Another combined fill application is disclosed in U.S. Pat. No. 4,317,785 to Dickey, Jr. et al. That patent describes a cooling tower with a number of film fill box-like sections arranged in a stair-step configuration progressing with the highest section at the outboard end of the fill area and the lowest section at the inboard end. The remainder of the tower available for water distribution is filled with splash fill. Air travels horizontally through the film fill boxes.
Crossflow fill sections and the framing to accommodate these sections (FIGS. 1-3) are normally standardized designs dimensioned in a cross-sectional parallelogram configuration which primarily depend upon the size of the cooling tower. This cross-sectional configuration basically conforms to the drift angle or trajectory of the water as it is released from the overhead water source 14 and is influenced by air (illustrated by arrow 15) flowing through the gas inlet opening horizontally at right angles to the falling water. Thus, this drift angle is primarily a function of the velocity of the entering air as it impinges on the falling water.
Contact of the released water with the film fill is very important considering the film fill has at least five times the cooling capability of the surrounding splash fill. Accordingly, any loss of film fill contact between water and air is critical, particularly at that low point in the tower where water must be at its coolest and if that portion of the water leaves the tower without any further chance to contact film fill.
In comparison, in cooling towers containing either all splash fill or all film fill, enough fill air travel can be provided to accommodate any variations in drift angle caused by varying cross air velocities so that all the released water will still contact the fill as it falls. In combined splash fill and sloped ('764 patent) or stepped ('785 patent) film fill cooling towers, however, the angle of slope and the resultant lower point of termination of the sloped film fill must be selected where the highest probability of impingement with the normal trajectory of water will occur. Hence, it is normally a compromise to attempt to establish a proper angle of water drift taking into account dimensions of the tower and the average cross air velocities.
The above-mentioned combined film fill/splash fill cooling tower arrangements perform exceptionally well when the drift angle falls within the design specifications. However, due to the adverse influence of the cross air flow on the descending released water, especially in fill sections of greater height, the drift angle often falls below or surpasses the designed drift angle. For example, as shown in FIG. 3, if the drift angle is less than design, the drift angle will be steeper and the lower water (illustrated by broken lines 17) will contact the film fill or splash fill outboard of the designed film fill section such that the lowermost cross flow air will not properly contact the water as it passes through the lowermost film fill. In contrast, if the cross-flow air velocity is higher than design, the lower water 17 may be carried more inwardly toward a tower plenum chamber 20 (FIG. 2) past the lowermost inward corner 21 of sloped film fill 11 (FIGS. 1 and 2). In both events, the overall cooling efficiency is reduced. Cross flow air velocity variations can be caused by several situations. Should the air flow be stopped or substantially slowed, such as in mechanical draft cooling towers to conserve fan power in the winter, the released water tends to fall more vertically having a drift angle smaller than the compromised design parameters. Further, wind blowing into one side of a tower can result in the descending water on the upwind side being carried too far inward toward the tower plenum chamber 20; while on the downwind side of the tower, the released water will not be carried far enough inward. In natural draft designs, where the air flow is induced by the chimney draft of a high discharge stack, winter drafts are higher than those of summer whereby the release water may be carried at a greater drift angle than that of the film fill section.
Since there is usually a limited amount of film fill in these combined splash fill/film fill cooling towers, for the reasons mentioned above, any loss of film fill contact between the water and the air is critical. This is particularly true at that low point in the cooling tower since this is the last region of heat exchange where the air and water can come into contact together at the film fill medium before that water falls into collecting basin 22.
Another problem associated with sloped film fill sections is that when they are too steeply slanted and too narrow, particularly if the cross flow air velocity is high, the fill tends to capture the water within the film fill so that the film of water may have difficultly running off a water discharge face or bottom edge of that film fill component. Consequently, the water retention tends to overload that fill and adversely affect cooling performance, as well as disturb the normal flow pattern of the water leaving the film fill, hence, adversely affecting the heat exchanging capabilities. The same thing can happen in a stepped film fill design where the steps must be overlapped due to the steep angle, and the overlapping creates a continuous path for the aforementioned retained water.
In an attempt to address this problem, U.S. Pat. No. 4,385,011 to Munters provides a film fill which more efficiently discharges the water from the face or bottom edge of the sloped film fill component. This solution, however, does not prevent the water retention and buildup occurring within the film fill.
By reducing the angle of slope of the film fill unit, the inner and lower film fill surfaces are permitted to more efficiently drain which substantially eliminates the above-mentioned water retention problem. This relatively flat film fill slope, however, may not match the framing of a standard tower, or may require special wider tower framing which increases the distance of the cross flow air flowing horizontally through the film fill section between gas inlet 16 and a gas outlet 18. Due to space and design limitations of the overall width of the fill unit, this configuration may preclude the lower inboard corner of a flatter film fill section from terminating at the desired lower inner corner of the overall tower fill section.